Many self-loading hand-operated firearms use locking arrangements which include a breech block carrier and a breech block head. The breech block carrier is movable in a defined path along the longitudinal axis of the weapon. The breech block head is carried by the breech block carrier for sliding movement and is rotatable about a longitudinal axis for locking and unlocking. The breech block head has a control pin on its rear portion which extends transversely to the longitudinal direction. The control pin is received in a sliding guide formed in the breech block carrier. The sliding guide extends obliquely to the longitudinal direction and is bounded by first and second guide edges.
During a shooting operation, the breech block carrier moves back and forth to cause ejection of a fired cartridge case and loading of a new cartridge. At the beginning of the opening movement of the breech block carrier, the breech block head is in its locked position. As the carrier moves rearward, the first guide edge engages and acts on the control pin to cause the breech block head to rotate to an unlocked position. The breech block head is then carried rearward by the breech block carrier. Shortly before the end of the forward closing movement of the breech block carrier, the breech block head engages the rear end of barrel and ceases movement along the longitudinal axis. The second guide edge of the sliding guide of the breech block carrier, which is still moving, contacts and acts on the control pin to cause the breech block head to rotate into its locked position. An example of firearms using this type of locking arrangement is the U.S. M16 automatic rifle.
Generally, the closing movement of the breech block carrier/head assembly should occur as rapidly as possible. This not only allows for a faster firing sequence but also causes cartridge being loaded to move as expediently as possible through a loading zone in which it is not completely guided. In this way, disturbances in the loading process are minimized such as in cases where the weapon is held in an oblique or even up-side-down position. The control pin, of course, cannot strike the end of the sliding guide so hard as to cause damage.
It is therefore desirable that the entire breech block assembly moves as quickly as possible in the closing movement over as much of its distance of travel as possible. Consequently, the angle that the sliding guide forms with the longitudinal direction is typically made very large so that the locking occurs in an end section of the closing movement that is as short as possible.
In many applications, such as military operations, hunting, or police actions, a firearm is often carried for a relatively long period of time in a state ready to be fired or loaded, without any opportunity for cleaning the weapon. In such a case, sand, dust or mud will unavoidably be deposited in the mechanisms of the weapon. Accordingly, the weapon may become jammed in the reloading process because the recoiling movement of the breech block assembly is hampered by the accumulated dirt.
Faulty ammunition may also cause the weapon to become jammed. For instance, inexperienced users tend to excessively oil the barrels of firearms. If a loaded cartridge is for some reason not entirely oil-tight, excessive oil can enter the cartridge and render a portion of the propellant ineffective and result in the so called "check shot." In such a case, the combustion gas of the faulty cartridge may be too weak to complete the recoiling movement of the breech block assembly.